DESCRIPTION (Applicant's abstract): Obstructive apneas and hypopneas occur during sleep in 3-5 percent of the population as a result of decrements in the activity of upper airway dilator muscles. The rapid eye movement (REM) state of sleep is characterized by postural atonia and the nadir of the activity in upper airway dilators. Two brainstem sites are critical for this depression of motor activity: 1) the medial pontine reticular formation (mPRF), where cholinergic mechanisms trigger the atonia and modulatory mechanisms contribute to its expression and maintenance; and 2) the medial medullary reticular formation controlling upper airway motoneurons and ascending neurons essential for the maintenance of the atonia. We hypothesize that cholinoceptive mPRF neurons are the target of modulatory effects exerted by aminergic and peptidergic inputs known to affect the expression of REM sleep atonia, send descending projections to the mMRF, and receive feedback pathways ascending from the mMRF. We further hypothesize that the projections descending from the mPRF target in the mMRF subpopulations of premotor neurons that control upper airway (hypoglossal - XII) motoneurons, local inhibitory interneurons whose function is to suppress the activity of medullary serotonin (5-HT)- containing neurons, and ascending neurons that terminate in the mPRF. To address this, in Specific Aim 1 we will determine which neurotransmitter receptors mediate effects in the mPRF neurons projecting to the mMRF. We will determine whether mRNA for muscarinic- and nicotinic-cholinergic, aminergic (5-HT1, 5HT2, alpha2), and peptidergic (VIP, CRF) receptors is present in individual mPRF cells which have been retrogradely labeled from the mMRF, dissociated, and subjected to the single-cell reverse transcription-polymerase chain reaction. In complementary whole cell clamping experiments, we will determine the effects of cholinergic, aminergic and peptidergic agonists on acutely dissociated cells of the mPRF having projections to the mMRF. In Specific Aim 2 we will use anterograde tracing and immunohistochemistry to determine whether axons of the cells located in the cholinoceptive region of the mPRF terminate on mMRF neurons containing glycine, GABA, met-enkephalin, or glutamate. In Specific Aim 3 we will combine anterograde and retrograde tracing to determine whether axons of the cells located in the mPRF terminate on mMRF cells that : a) are premotor to XII motoneurons; and b) have axons ascending to the mPRF, therefore providing the feedback needed to maintain the REM sleep atonia. In Specific Aim 4 we will identify the transmitters present in XII premotor neurons contacted by axon terminals descending from the mPRF using fluorescent labeling and confocal microscopy. The proposed studies will identify the neurochemistry of the pontomedullary pathway responsible for the REM sleep atonia of upper airway motoneurons at the molecular, cellular and network level.